Altering Text Size based on Category Popularity

Hopefully a quick one, I’m sure I just need to pointed in the right direction.

I’d like my text of my category/tag links to be related in size to the number of posts on the topic. Which is, I assume, how the tags/categories of the forum are styled (it’s not random is it?). There must be advice on how to do it, but I can’t find the right search terms.